The invention relates to a method and a device for controlling the operating point of a battery, in particular a secondary battery, for a motor vehicle, in particular for the purpose of automatically setting the operating point. A secondary battery is considered to be a re-chargeable storage unit (also known as a storage battery or secondary storage unit), as opposed to a primary battery which cannot be re-charged. The word “battery” as used below refers in all cases to a secondary battery. In particular, a nickel metal hybrid battery, a lithium ion battery, a lead battery or a lithium polymer battery or another suitable, re-chargeable storage unit is used in particular as a motor vehicle battery. The operating point of the battery is here in particular considered to be the setting of the charging state of the battery in which an optimum, i.e. a maximum, working life-extending, input or removal of power is possible.
Currently, ever increasing demands are made on vehicle batteries due to the increasing number of electrical systems and the addition of systems which also need to be supplied when the motor vehicle is switched off. Thus, diesel engines require high currents, in particular at low temperatures (high cold-start power) and motor vehicles with electric equipment such as electro-hydraulic brake systems require constant battery availability for safety reasons.
Here, the capacity, i.e. the quantity of current which can be removed under certain conditions, depends strongly on non-electrical parameters such as ambient temperature, cell temperature and age. Of particular significance is the strongly temperature-dependent behaviour of a battery. Thus for example, the capacity and the charge state of a nickel metal hydride battery (also abbreviated to NiMH battery) depend strongly on the ambient temperature due to the hydrogen storage alloy used in the negative electrode. At low temperatures, hydrogen with worse dismantling and installation kinetics is emitted and absorbed by the negative electrode. At higher temperatures, the hydrogen absorption and thus the chargeability is noticeably negatively influenced by the negative electrode, since with the increasing temperature, a hydrogen counter-pressure is built up. These temperature-dependent effects occur with modern, metal hydride storage units (also known as “hydrogen tanks”) with which hydrogen is well bonded in a hydrogen storage alloy, but is also expelled again by heat. As a result, the capacity and therefore the removal are restricted.
Furthermore, the capacity, i.e. the removable charge or current quantity, depends strongly on the charging state of the battery. Modern batteries such as nickel metal hydride batteries or lithium ion batteries comprise a high power density of up to 1.8 kw per kg or 3 kW per kg, so that a high level of removal and good chargeability at low temperatures is already partially possible.
In order to be able to retain the different electrical functions of the motor vehicle while driving, such as the start function or safety functions, the battery availability is usually monitored constantly. In this regard, it is known how to determine the battery state, e.g. the charging state, the capacity and/or the age of the battery and from this information, to determine the current power rating of the battery. Here, methods of this type are generally based on expensive current and voltage measuring procedures, or on expensive model calculations based on experience.
It is thus known from DE 199 18 529 B4 how to determine the charging state and/or the capacity of a battery based on a measurement of the battery current and a calculation of a measured removed charge from the time progression of the battery current, taking into account the open-circuit voltage of the battery.
The known methods here enable the determination of the removable charge (=discharging) and the charging ability (=charging) of the battery in its capacity range, based on the current and voltage measurement, and taking into account the charging state of the battery. Thus, a nickel metal hydride battery is able to operate with full charging and discharging power in a capacity range of between 30% and 70%, due to the high power density. When the battery has reached a charging state of 90%, the charging is strongly restricted, since otherwise, harmful overcharging of the battery may occur, which could lead to a significant reduction in the working life of the battery.
The object of the invention is to provide a method for controlling the operating point of a battery, so that an optimal utilisation of the capacity range of the battery is made possible. The object is also to provide a particularly suitable device.